1. Field of the Invention
The present invention relates to a structure for mounting roof carriers on the roof of a motorcar and, more particularly, to a structure for detachably mounting roof carriers on the roof of a motorcar in which fixing portions at which mounting brackets of the roof carriers are fixed are directly formed on the car roof.
2. Description of the Prior Art
In recent years, the surface of a car body has been designed to become as aerodynamic as possible in order to reduce an air resistance coefficient. As a result of the latest in such developments, a car roof as shown in FIG. 1 which is known as a "Mohican" roof has been often employed.
The Mohican roof has roof side rails 2 forming an outer shell of the car body roof 1 and respectively extend in the longitudinal direction of the car on both sides of the car roof 1, and a main roof panel 3 spanning both roof side rails 2 so as to extend in the central area of the car body, both side edges of the main roof panel 3 being welded to the inner edges 2a of the roof side rails 2. One of the external features of the Mohican roof is that on the sides of the car body roof 1 there are not any weather strips which, if provided, would project further outwardly of the roof side rail 2 for the purpose of draining water such as rain water or the like.
Accordingly, as far as the Mohican roof is concerned, a mounting bracket 4a of a roof carrier 4 cannot be detachably fixed to the roof when a weather strip is used. In view of such inconvenience, a fixing portion 5 for the mounting bracket 4a of the roof carrier 4 has been provided on the car body roof 1 in order to assure that mounting bracket 4a can be detachably fixed directly to the fixing portion 5.
Specifically, as shown in FIG. 1, a joint 6 at which the inner edge 2a of the roof side rail 2 is joined to the side edge 3a of the main roof panel 3 is depressed in the longitudinal direction of the car body to form a longitudinally extending groove 7, and the mounting bracket 4a of the roof carrier 4 is placed on the bottom surface defining the longitudinal groove 7. A hole 9 is drilled through the joint 6 in alignment with the location at which the foot 4a of the roof carrier 7 is disposed, and a nut 10 is welded on the lower surface of the joint 6 in alignment with the hole 9 to form the fixing portion 5. Thus, when the roof carrier 4 is mounted on the car body roof, the foot 4a of the roof carrier 7 is fastened directly onto the car roof 1 with the use of a bolt 8.
As mentioned above, the roof side rail 2 is a structural member (side member) of the car body and it includes a hollow space which is formed in a manner in which a roof side outer panel 11 is fixedly secured to a roof side inner panel 12, their inner ends 11a and 12a as well as at their outer ends 11b and 12b being superposed.
Accordingly, the joint 6 at which the roof side rail 2 is joined to the main roof panel 3 is constructed so that the inner end 11a of the roof side outer panel 11 is superposed on the inner end 12a of the roof side inner panel 12 and the side edge 3a of the main roof panel 3 is superposed on the inner end 11a of the roof side outer panel 11. These three panels 3, 11 and 12 are welded together one above the other when the car body roof 1 is assembled.
On the other hand, the fixing portion 5 which is to extend in the longitudinal groove 7 of the joint portion 6 is not formed after the car roof 1 is assembled but is formed in a manner in which the hole 9 through which the bolt 8 is inserted is previously formed through each of the panels 3, 11 and 12, the nut 10 is welded to the lower surface of the roof side inner panel 12, the hole 9 on each of the panels 3, 11 and 12 is aligned with the nut 10 when the car body roof 1 is assembled, and the three panels 3, 11 and 12 are then welded together. Since the fixing portion 5 for detachably fixing the foot 4a of the roof carrier 4 is formed in the above-described manner, the degree of accuracy required for positioning the three members of the panels 3, 11 and 12 is relatively high. Consequently, the operational efficiency of assembling the car body roof 1 is relatively low and thus productivity, is also relatively low.
Furthermore, when the roof carrier 4 is mounted, a load exerted by the dead weight of the roof carrier 4 is concentrated on the fixing portion 5 in the longitudinal groove 7 via the mounting bracket 4a, causing a peripheral portion surrounding the fixing portion 5 to be distorted. Thus, there a problem arises with respect to the strength of the car roof 1.
Incidentally, the present assignee has proposed, as disclosed in Japanese Laid-Open Utility Model No. 39245/1984 entitled "Roof Carrier Device for a Motorcar", that a carrier base member be fixedly provided substantially over the entire length of the longitudinal groove with the mounting bracket of the roof carrier fixed to the carrier base member. Although this proposed structure assures sufficient strength, a long carrier base member is required as a reinforcement member. Furthermore, the operational efficiency of assembling of the car body roof still remains relatively low.